Space-time processing for wireless systems with multiple transmit and receive antennas

ABSTRACT

Signals are developed for use in a wireless system with multiple transmit and multiple receive antennas so that even in the face of some correlation the most open-loop capacity that can be achieved using a substantially open-loop system with a channel of that level of correlation is obtained. In accordance with the principles of the invention, the signals transmitted from the various antennas are processed so as to improve their ability to convey the maximum amount of information. More specifically, the data to be transmitted is divided into M+1 substreams, where M is the number of transmit antennas. Each transmit antenna is supplied with a combination signal that is made up of a weighted version of a common one of the substreams and a weighted version of a respective one of the substreams that is supplied uniquely for that antenna, so that there are M transmit signals. A receiver having N antennas receives the M transmit signals as combined by the channel and reconstitutes the original data therefrom. This may be achieved using successive decoding techniques. Advantageously, the capacity, i.e., the rate of information that can be conveyed with an arbitrarily small probability of error when the instantaneous forward channel condition is unknown to the transmitter, is maximized.

TECHNICAL FIELD

[0001] This invention relates to the art of wireless communications, andmore particularly, to wireless communication systems using multipleantennas at the transmitter and multiple antennas at the receiver, socalled multiple-input multiple-output (MIMO) systems.

BACKGROUND OF THE INVENTION

[0002] It is well known in the art that multiple-input multiple-output(MIMO) systems can achieve dramatically improved capacity as compared tosingle antenna, i.e., single antenna to single antenna or multipleantenna to single antenna, systems. However, to achieve thisimprovement, it is preferable that there be a rich scatteringenvironment, so that the various signals reaching the multiple receiveantennas be largely uncorrelated. If the signals have some degree ofcorrelation, and such correlation is ignored, performance degrades andcapacity is reduced.

SUMMARY OF THE INVENTION

[0003] We have invented a way of developing signals in a MIMO systemsuch that even in the face of some correlation the most open-loopcapacity that can be achieved using a channel of that level ofcorrelation is obtained. In accordance with the principles of theinvention, the signals transmitted from the various antennas areprocessed so as to improve their ability to convey the maximum amount ofinformation. More specifically, the data to be transmitted is dividedinto M+1 substreams, where M is the number of transmit antennas. Eachtransmit antenna is supplied with a combination signal that is made upof a weighted version of a common one of the substreams and a weightedversion of a respective one of the substreams that is supplied uniquelyfor that antenna, so that there are M transmit signals. A receiverhaving N antennas receives the M transmit signals as combined by thechannel and reconstitutes the original data therefrom. This may beachieved using successive decoding techniques. Advantageously, theopen-loop capacity, i.e., the rate of information that can be conveyedwith an arbitrarily small probability of error when the instantaneousforward channel condition is unknown to the transmitter, is maximized.

[0004] In one embodiment of the invention, the weights are determined bythe forward channel transmitter using channel statistics of the forwardlink which are made known to the transmitter of the forward link bybeing transmitted from time to time from the receiver of the forwardlink by the transmitter of the reverse link. In another embodiment ofthe invention, a determination of weight parameter, or the weightsthemselves, is made by the forward channel receiver using the channelstatistics of the forward link and the determined weight parameter, orweights, is made known to the transmitter of the forward link by beingtransmitted from time to time from the receiver of the forward link bythe transmitter of the reverse link.

BRIEF DESCRIPTION OF THE DRAWING

[0005] In the drawing:

[0006]FIG. 1 shows an exemplary portion of a transmitter for developingsignals to transmit in a MIMO system having a transmitter with Mtransmit antennas transmitting over a forward channel, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained, inaccordance with the principles of the invention;

[0007]FIG. 2 shows an exemplary portion of a receiver for a MIMO systemarranged in accordance with the principles of the invention;

[0008]FIG. 3 shows an exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system such that even in theface of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention;

[0009]FIG. 4 shows another exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system such that even in theface of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention; and

[0010]FIG. 5 shows an another exemplary portion of a transmitter fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel, such thateven in the face of some correlation the most open-loop capacity thatcan be achieved with a channel of that level of correlation is obtained,in accordance with the principles of the invention.

DETAILED DESCRIPTION

[0011] The following merely illustrates the principles of the invention.It will thus be appreciated that those skilled in the art will be ableto devise various arrangements which, although not explicitly describedor shown herein, embody the principles of the invention and are includedwithin its spirit and scope. Furthermore, all examples and conditionallanguage recited herein are principally intended expressly to be onlyfor pedagogical purposes to aid the reader in understanding theprinciples of the invention and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the invention, as well as specific examples thereof, areintended to encompass both structural and functional equivalentsthereof. Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

[0012] Thus, for example, it will be appreciated by those skilled in theart that the block diagrams herein represent conceptual views ofillustrative circuitry embodying the principles of the invention.Similarly, it will be appreciated that any flow charts, flow diagrams,state transition diagrams, pseudocode, and the like represent variousprocesses which may be substantially represented in computer readablemedium and so executed by a computer or processor, whether or not suchcomputer or processor is explicitly shown.

[0013] The functions of the various elements shown in the FIGS.,including functional blocks labeled as “processors” may be providedthrough the use of dedicated hardware as well as hardware capable ofexecuting software in association with appropriate software. Whenprovided by a processor, the functions may be provided by a singlededicated processor, by a single shared processor, or by a plurality ofindividual processors, some of which may be shared. Moreover, explicituse of the term “processor” or “controller” should not be construed torefer exclusively to hardware capable of executing software, and mayimplicitly include, without limitation, digital signal processor (DSP)hardware, read-only memory (ROM) for storing software, random accessmemory (RAM), and non-volatile storage. Other hardware, conventionaland/or custom, may also be included. Similarly, any switches shown inthe FIGS. are conceptual only. Their function may be carried out throughthe operation of program logic, through dedicated logic, through theinteraction of program control and dedicated logic, or even manually,the particular technique being selectable by the implementor as morespecifically understood from the context.

[0014] In the claims hereof any element expressed as a means forperforming a specified function is intended to encompass any way ofperforming that function including, for example, a) a combination ofcircuit elements which performs that function or b) software in anyform, including, therefore, firmware, microcode or the like, combinedwith appropriate circuitry for executing that software to perform thefunction. The invention as defined by such claims resides in the factthat the functionalities provided by the various recited means arecombined and brought together in the manner which the claims call for.Applicant thus regards any means which can provide those functionalitiesas equivalent as those shown herein.

[0015]FIG. 1 shows an exemplary portion of a transmitter for developingsignals to transmit in a MIMO system having a transmitter with Mtransmit antennas transmitting over a forward channel, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained, inaccordance with the principles of the invention. Shown in FIG. 1 are a)demultiplexer (demux) 101; b) weight supplier 105; c) antennas 107,including antennas 107-1 through 107-M; d) adders 109, including adders109-1 through 109-M; e) multipliers 111-1 through 111 -M+1; and f) radiofrequency (RF) converters 115; including 115-1 through 115-M.

[0016] Demultiplexer 101 takes a data stream as an input and supplies asan output M+1 data substreams by supplying various bits from the inputdata stream to each of the data substreams. The data substreams aresupplied by demultiplexer 101 to a respective one of multipliers 111.Multiplier 111-1 through 111-M multiply each value of the first M datasubstreams by a first weight supplied by weight supplier 105. Typically,each of the first M weighted data substreams are of equal rate.Similarly, multiplier 111-M+1 multiplies each value of the M+1^(th) datasubstream by a second weight supplied by weight supplier 105.

[0017] Typically the M+1^(th) data substream is not at the same rate asthe first M data substreams. As will be recognized by those of ordinaryskill in the art, the particular rates for the first M data substreamsand the M+1^(th) data substream are dependent on the receiver, inparticular, the order in which the receiver performs the successivedecomposition. Thus, the particular rates are typically negotiated fromtime to time between the receiver and the transmitter. Note that themore correlated the channel is, the larger the rate of the M+1^(th) datasubstream.

[0018] The first and second weights may be related to each other, andmay be developed by weight supplier 105 from a common weight parameterwhich may be derived from statistics of the forward channel, as will bedescribed in more detail hereinbelow. In one embodiment of theinvention, weight supplier 105 actually develops the weight values inresponse to information received via the reverse channel from thereceiver shown and described further in FIG. 2. In another embodiment ofthe invention the weight values are developed in the receiver, thensupplied via the reverse channel to the transmitter, in which they arestored in weight supplier 105 until such time as they are required. Aprocess for developing the weights in accordance with an aspect of theinvention will be described hereinbelow.

[0019] Each of the first M weighted data substreams is supplied as aninput of a respective one of adders 109. Each of adders 109 alsoreceives at its other input the weighted M+1^(th) data substream whichis supplied as an output by multiplier 111-M+1. Each of adders 109combines the two weighted data substreams input to it so as to produce acombined branch signal. Thus, M combined branch signal are produced, oneby each of adders 109. Each of radio frequency (RF) converters 115receives one of the M combined branch signals and develops therefromradio frequency versions of the M combined branch signals, which arethen supplied to respective ones of antennas 107 for transmission.

[0020]FIG. 2 shows an exemplary portion of a receiver for a MIMO systemarranged in accordance with the principles of the invention. FIG. 2shows a) N antennas 201, including antennas 201-1 through 201-N; b)radio frequency (RF) converters 203, including radio frequency (RF)converters 203-1 through 203-N; c) channel statistics estimation unit207; e) optional weight parameter calculator 209; and f) optional switch211. Each of antennas 201 receives radio signals and supplies anelectrical version thereof to its respective, associated one of radiofrequency (RF) converters 203. Each of radio frequency (RF) converters203 downconverts the signal it receives to baseband, converts thebaseband analog signal it received to a digital representation, andsupplies the digital representation to channel statistics estimationunit 207.

[0021] Channel statistics estimation unit 207 develops certainstatistics regarding the channel. In particular, channel statisticsestimation unit 207 may develop a) an estimate of the averagesignal-to-interference-and-noise ratio (SINR), ρ, and b) the correlationamong the channel components, η. The correlation among the channelcomponents is developed using an estimate of the forward matrix channelresponse which is developed in the conventional manner. Note thatmatrices are required because there are multiple transmit antennas andmultiple receive antennas. More specifically, the correlation among thechannel components over a time period may be computed as η=K/(K+1),where K is the well known Ricean spatial K-factor

[0022] The channel statistics are supplied either to optional weightparameter calculator 209 or they are supplied via the reverse channel tothe transmitter (FIG. 1). If the channel statistics are supplied toweight parameter calculator 209, weight parameter calculator 209determines the weight parameter that is to be used, in accordance withan aspect of the invention and as described hereinbelow, and suppliesthe resulting weight parameter to the transmitter (FIG. 1) via thereverse channel.

[0023]FIG. 3 shows an exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel to areceiver having N receiver antennas and a reverse channel forcommunicating from the receiver to the transmitter, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention. The process of FIG. 3 may be employed in an embodiment ofthe invention that uses the hardware of FIGS. 1 and 2, with switch 211being connected to channel statistics estimation unit 207 as follows.

[0024] First it is necessary to determine the length of time duringwhich the channel statistics are stable. This is typically performed atthe system engineering phase of developing the system, usingmeasurements of the environment into which the system is to be deployed,as is well known by those of ordinary skill in the art. Once the lengthof time for which the channel statistics are stable is known, that timeis the time period over which information will be gathered to generateeach statistic.

[0025] The process of FIG. 3 is entered in step 301 at the beginning ofeach time period. Next, in step 303, the channel statistics areestimated over the time period.

[0026] Thereafter, in step 305, (FIG. 3) the statistics are supplied bythe receiver of the forward link to the transmitter of forward link,e.g., via the reverse channel.

[0027] In step 307 the first and second weights, α₁ and α₂ arecalculated, e.g., by weight supplier 105 (FIG. 1). More specifically,the weights are calculated as follows. $\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

[0028] where M, N, ρ, η are as defined hereinabove and P_(T) is thetotal available transmit power. Thus it can be seen that there is arelationship between the two weights, allowing one of them to act as theweight parameter from which the other is determined, e.g., according tothe following M(α₁² + α₂²) = P_(T).

[0029] In step 309, the input data stream is divided into M+1 substreamse.g., by demultiplexer 101 (FIG. 1). Each of the first M data substreamsis then multiplied by weight α₁ in step 311 (FIG. 3). In other words,each bit of each particular data stream is multiplied by α₁ to produce Mweighted data substreams. Additionally, the M+1^(th) data substream ismultiplied by α₂ to produce the M+1^(th) weighted data substream.

[0030] In step 313, each of the first M weighted data substreams iscombined with the M+1^(th) weighted data substream, e.g., by adders 109.The process then exits in step 315.

[0031]FIG. 4 shows another exemplary process, in flow chart form, fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel to areceiver having N receiver antennas and a reverse channel forcommunicating from the receiver to the transmitter, such that even inthe face of some correlation the most open-loop capacity that can beachieved with a channel of that level of correlation is obtained with asubstantially open-loop process, in accordance with the principles ofthe invention. The process of FIG. 4 may be employed in an embodiment ofthe invention that uses the hardware of FIGS. 1 and 2, with switch 211being connected to weight calculator 209. Note that for the process ofFIG. 4, weight supplier 105 of FIG. 1 will not compute the variousweights, but will instead merely store the weights received from weightcalculator 209 and supply them to the various ones of multipliers 113 asis necessary.

[0032] The process of FIG. 4 is entered in step 401 at the beginning ofeach time period. Next, in step 404, the channel statistics areestimated over the time period.

[0033] In step 405 at least one of the weights α₁ and α₂ are calculatede.g., by weight parameter calculator 209 (FIG. 2). The at least oneweight, or both of the weights, if both are calculated, are calculatedin the same manner as described above. It is only necessary to calculateone of the weights which can then act as the weight parameter, fromwhich the other weight can be determined in the transmitter using therelationship described above.

[0034] Thereafter, in step 407, either both weights or the determinedweight parameter is supplied by the receiver of the forward link to thetransmitter of forward link, e.g., via the reverse channel. The weightis stored in weight supplier 105 (FIG. 1). If only one weight issupplied as a weigh parameter, the other weight is computed in weightsupplier 105 and then also stored therein.

[0035] In step 409, the input data stream is divided into M+1 substreamse.g., by demultiplexer 101 (FIG. 1). Each of the first M data substreamsis then multiplied by weight α₁ in step 411 (FIG. 4). In other words,each bit of each of each particular data stream is multiplied by α₁ toproduce M weighted data substreams. Additionally, the M+1^(th) datasubstream is multiplied by α₂ to produce the M+1^(th) weighted datasubstream.

[0036] In step 413 each of the first M weighted data substreams iscombined with the M+1^(th) weighted data substream, e.g., by adders 109.The process then exits in step 415.

[0037]FIG. 5 shows an another exemplary portion of a transmitter fordeveloping signals to transmit in a MIMO system having a transmitterwith M transmit antennas transmitting over a forward channel, such thateven in the face of some correlation the most open-loop capacity thatcan be achieved with a channel of that level of correlation is obtained,in accordance with the principles of the invention. Shown in FIG. 5 area) demultiplexers (demux) 501 and 503; b) weight supplier 505; c)antennas 507, including antennas 507-1 through 507-M; d) adders 509,including adders 509-1 through 509-M; e) multipliers 511-1 and 511-2;and f) radio frequency (RF) converters 515; including 515-1 through515-M.

[0038] Demultiplexer 501 takes a data stream as an input and supplies asan output two data substreams by supplying various bits from the inputdata stream to each of the data substreams. The first data substream issupplied by demultiplexer 501 to multiplier 511-1 while the second datasubstream is supplied to multiplier 511-2. Multiplier 511-1 multiplieseach value of the first substream by a first weight supplied by weightsupplier 505. Similarly, multiplier 511-2 multiplies each value of thesecond substream by a second weight supplied by weight supplier 505.

[0039] The first and second weights may be related to each other, andmay be developed by weight supplier 505 from a common weight parameterwhich may be derived from statistics of the forward channel, as will bedescribed in more detail hereinbelow. In one embodiment of theinvention, weight supplier 505 actually develops the weight values inresponse to information received via the reverse channel from thereceiver shown and described herein above in connection with FIG. 2. Inanother embodiment of the invention the weight values are developed inthe receiver, then supplied via the reverse channel to the transmitter,in which they are stored in weight supplier 505 until such time as theyare required.

[0040] Demultiplexer 503 takes the weighted data substream supplied asan output by multiplier 511-1 and supplies as an output M weighted datasubstreams by supplying various bits from the weighted data substream itreceived to each of the data M weighted substreams. Typically, each ofthe M weighted data substreams are of equal rate. Each of the M weighteddata substreams developed by demultiplexer 503 is supplied as an inputof a respective one of adders 509. Each of adders 509 also receives atits other input the weighted second substream which is supplied as anoutput by multiplier 511-2. Each of adders 509 combines the two weighteddata substreams input to it so as to produce a combined branch signal.Thus, M combined branch signal are produced, one by each of adders 509.Each of radio frequency (RF) converters 515 receives one of the Mcombined branch signals and develops therefrom radio frequency versionsof the M combined branch signals, which are then supplied to respectiveones of antennas 507 for transmission.

[0041] In another embodiment of the invention, for use with so-called“time division duplex” (TDD) systems, which share a single channel forboth the forward and reverse channels, the calculation of thecorrelation among the channel components η may be performed at eitherend of the wireless link. This is because, since the forward and reversechannels share the same frequency channel, alternating between which isusing the channel at any one time, the channel statistics for theforward and reverse channels will be the same. Therefore, the receiverof the reverse channel will experience the same correlation among thechannel components η as the receiver of the forward channel, and so thereceiver of the reverse link can measure the correlation among thechannel components η that was previously measured by the receiver of theforward link. Likewise, the receiver of the forward channel willexperience the same channel response as the receiver of the reversechannel, and so the receiver of the forward link can determine thecorrelation among the channel components η that were previouslydetermined by the receiver of the reverse link. However, the SINR muststill be computed only at the receiver and relayed to the transmitter ifnecessary.

What is claimed is:
 1. A method for transmitting a data signal incommunications over a forward channel, the method comprising the stepsof: demultiplexing said data signal into M+1 data substreams, M≧2;weighting the first M of said data substreams with a first weight toproduce M first weighted substreams; weighting the remaining M+1^(th)data substream with a second weight to produce one second weighted datasubstream; combining each respective one of said M first weightedsubstreams with said second weighted data substream to produce Mcombined weighted data substreams.
 2. The invention as defined in claim1 further comprising the step of transmitting each of said combinedweighted data substreams from a respective one of M transmit antennas.3. The invention as defined in claim 1 further comprising the step ofreceiving a weight parameter via a reverse channel and developingtherefrom said first and second weights.
 4. The invention as defined inclaim 1 wherein said first and second weights are determined as afunction of forward channel statistics received from a receiver via areverse channel.
 5. The invention as defined in claim 1 furtherincluding the step of converting said combined weighted data substreamsinto radio frequency signals.
 6. The invention as defined in claim 1wherein said first and second weights are determined by solving$\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 7. The invention as defined inclaim 6 wherein ${\eta = \frac{K}{K + 1}},$

where K is the well known Ricean spatial K factor.
 8. Apparatus fortransmitting a data signal in a communications system over a forwardchannel, the apparatus comprising: means for demultiplexing said datasignal into M+1 data substreams, M≧2; means for weighting the first M ofsaid data substreams with a first weight to produce M first weightedsubstreams; means for weighting the remaining M+1^(th) data substreamwith a second weight to produce one second weighted data substream;means for combining each respective one of said M first weightedsubstreams with said second weighted data substream to produce Mcombined weighted data substreams.
 9. The invention as defined in claim8 wherein said apparatus comprises means for developing said weights.10. The invention as defined in claim 8 wherein said apparatus comprisesmeans for storing said weights.
 11. The invention as defined in claim 8wherein said apparatus further comprises: means receiving a weightparameter via a reverse channel; and means for developing said first andsecond weights from said weight parameter.
 12. The invention as definedin claim 8 further comprising means for transmitting each of saidcombined weighted data substreams as a radio frequency signal from arespective one of M transmit antennas.
 13. The invention as defined inclaim 8 wherein said first and second weights are determined by meansfor solving $\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 14. A transmitter for transmittinga data signal in communications system over a forward channel, thetransmitter comprising: a demultiplexer that divides said data signalinto M+1 data substreams, M≧2; multipliers for weighting the first M ofsaid data substreams with a first weight to produce M first weightedsubstreams; multipliers for weighting the remaining M+1^(th) datasubstream with a second weight to produce one second weighted datasubstream; adders for combining each respective one of said M firstweighted substreams with said second weighted data substream to produceM combined weighted data substreams.
 15. The invention as defined inclaim 14 further comprising a radio frequency converter for convertingeach of said combined weighted substreams to radio frequency forbroadcast by a respective one of M transmit antennas.
 16. The inventionas defined in claim 14 wherein said weights are determined in saidtransmitter in response to a weight parameter received from a receiverover a reverse channel.
 17. The invention as defined in claim 14 whereinsaid weights are determined in said transmitter in response to channelstatistics received over a reverse channel from a receiver.
 18. Theinvention as defined in claim 14 wherein said wherein said first andsecond weights are determined by solving $\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 19. The invention as defined inclaim 14 wherein said transmitter and receiver communicate using timedivision duplexing (TDD) and said weights are determined in saidtransmitter using an estimate of the correlation among the channelcomponents that is determined by a receiver of a reverse link for saidtransmitter.
 20. A receiver for use in a MIMO system, comprising: Nforward channel receive antennas; N radio frequency (RF) converters,each RF converter downconverting a signal it receives from a respectiveassociated one of said N antennas to an analog baseband signal andconverting said analog baseband signal to a digital representation; anestimator, responsive to said digital representations, for estimatingchannel statistics for a forward channel being received by saidreceiver; and a transmitter for a reverse channel for transmitting saidchannel statistics from time to time to a receiver for said reversechannel.
 21. A receiver for use in a MIMO system, comprising: anestimator, responsive to N digital representations of signals of aforward channel received by N respective antennas, for determining anestimate of the average signal-to-interference-and-noise ratio (SINR)for said forward channel being received by said receiver; an estimator,responsive to N digital representations of signals of a forward channelrecited by N respective antennas, for determining an estimate of acorrelation among the channel components for said forward channel beingreceived by said receiver; and a transmitter for transmitting over areverse channel said estimate of the average SINR and said estimate of acorrelation among the channel components.
 22. A receiver for use in aMIMO system, comprising: an estimator, responsive to N digitalrepresentations of signals of a forward channel received by N respectiveantennas for determining an estimate of the averagesignal-to-interference-and-noise ratio (SINR) for a forward channelbeing received by said receiver; an estimator for determining anestimate of a correlation among the channel components for a forwardchannel being received by said receiver; and a weight calculator forcalculating at least one weight for use by a transmitter of said forwardchannel to transmit data substreams to said receiver as a function ofsaid estimates of SINR and correlation among the channel components 23.The invention as defined in claim 22 further including a transmitter fora reverse channel for transmitting said at least one weight to areceiver for said reverse channel.
 24. A receiver for use in a MIMOsystem, comprising: N receive antennas; N radio frequency (RF)converters, each RF converter downconverting a signal it receives from arespective associated antenna to an analog baseband signal andconverting said analog baseband signal to a digital representation; anestimator, responsive to said digital representations, for determiningan estimate of the average signal-to-interference-and-noise ratio (SINR)for a forward channel being received by said receiver; an estimator fordetermining an estimate of a correlation among the channel componentsfor a forward channel being received by said receiver; and a weightcalculator for calculating at least one weight for use by a transmitterof said forward channel to transmit data substreams to said receiver asa function of said estimates of SINR and correlation among the channelcomponents, said at least one weight being determined in said weightcalculator by solving at least one equation of the set consisting of$\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are first and second weights, respectively, P_(T) is thetotal available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 25. A method for transmitting adata signal in communications over a forward channel, the methodcomprising the steps of: demultiplexing said data signal into two datasubstreams; weighting the first of said two data substreams with a firstweight to produce a first weighted substream; weighting the second ofsaid two data substreams with a second weight to produce a secondweighted substream; demultiplexing said second weighted data substreaminto M weighted data substreams, M≧2; combining each respective one ofsaid M weighted substreams with said first weighted data substream toproduce M combined weighted data substreams.
 26. The invention asdefined in claim 25 further comprising the step of transmitting each ofsaid combined weighted data substreams from a respective one of Mtransmit antennas.
 27. The invention as defined in claim 25 wherein saidfirst and second weights are determined by solving $\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 28. Apparatus for transmitting adata signal in a communications system over a forward channel, theapparatus comprising: means for demultiplexing said data signal into twodata substreams; means for weighting the first of said two datasubstreams with a first weight to produce a first weighted substream;means for weighting the second of said two data substreams with a secondweight to produce a second weighted substream; means for demultiplexingsaid second weighted data substream into M weighted data substreams,M≧2; means for combining each respective one of said M weightedsubstreams with said first weighted data substream to produce M combinedweighted data substreams.
 29. The invention as defined in claim 28further comprising means for transmitting each of said combined weighteddata substreams as a radio frequency signal from a respective one of Mtransmit antennas.
 30. The invention as defined in claim 28 wherein saidfirst and second weights are determined by means for solving$\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas.
 31. A transmitter for transmittinga data signal in communications system over a forward channel, thetransmitter comprising: a first demultiplexer that divides said datasignal into two data substreams; multipliers for weighting the first ofsaid two data substreams with a first weight to produce a first weightedsubstream; multipliers for weighting the second of said two datasubstreams with a second weight to produce a second weighted substream;a second demultiplexer that divides said second weighted data substreaminto M weighted data substreams, M≧2; M adders for combining eachrespective one of said M weighted substreams with said first weighteddata substream to produce M combined weighted data substreams.
 32. Theinvention as defined in claim 31 further comprising the step oftransmitting each of said combined weighted data substreams from arespective one of M transmit antennas.
 33. The invention as defined inclaim 31 wherein said first and second weights are determined by solving$\begin{matrix}{\alpha_{1} = \sqrt{\frac{P_{T}}{M} - \alpha_{2}^{2}}} \\{\alpha_{2} = \sqrt{\frac{P_{T}\eta}{\rho \quad {N\left( {1 - \eta} \right)}\left( {{M\quad \eta} + 1 - \eta} \right)}}}\end{matrix}$

where α₁ and α₂ are said first and second weights, respectively, P_(T)is the total available transmit power ρ is an estimate of the averagesignal-to-interference-and-noise ratio (SINR), and η is the correlationamong the channel components, M is the number of transmit antennas, andN is the number of receiver antennas